An alkaline earth metal phenate is one of the alkaline detergents for use in lubricating oils for mainly internal combustion engines, which functions to neutralize and disperse acid substances, sludge, etc. generated in engines. The existence of an alkaline earth metal phenate makes it possible to prevent engines parts from excessive corroding caused by acid substances generated in engines and prevent engine parts from excessive wearing caused by sludge or lacquer generated engines. Recently, in order to have high acid-neutralizing ability, over-based alkaline earth metal phenates have been developed (a phenate which contains an alkaline earth metal in an amount of more than the theoretical amount per an alkylphenol). At present, sulfurized types have been mainly used to improve heat resistance and oil solubility. Furthermore, in order to obtain an alkaline earth metal phenate having higher over-basicity, in some cases carboxylic acids or alcohols are added to alkaline earth metal phenates.
Insofar as typical conventional methods for introducing sulfur into phenates are concerned, two methods are generally known. One of them is a method wherein an alkylphenol is treated with sulfur chloride to form a sulfide which is then converted into a metal salt. The other is a method wherein sulfurization is carried out by using elemental sulfur during metal addition or after previously forming a metal salt. The present invention belongs to the latter sulfurization method. Many past attempts have been made to produce over-based phenates by using the latter sulfurization method, for instance, it is reported that over-based phenates containing alkaline earth metals in an amount of at least twice the theoretical amount are obtained by one metal addition reaction.
For example, JP-B-60-28878 discloses that highly basic alkaline earth metal phenates can be unexpectedly produced by carrying out the above-mentioned reaction in the presence of an excess of phenols. (The term "JP-B" as used herein means an "examined Japanese patent publication".)
Further, JP-B-3-10611 discloses reacting a phenol, a dihydric alcohol, an alkaline earth metal reagent and optionally sulfur to conduct a metal addition of phenols which may be conducted with sulfurization, whereby the conversion per one reaction of alkaline earth metal reagents into products can be remarkably improved by the addition of a suitable amount of water without the necessity of an excess amount of dihydric alcohols.
In contrast to a conventional method in which a large excess amount of alkaline earth metal reagents is used per an alkylphenol to provide an over-based product, these two methods succeed in providing an over-based phenate by using a large excess amount of alkylphenols per an alkaline earth metal reagent, which makes it possible to use all materials, except for mineral oil as a diluent oil, which are directly concerned with the reaction system as reaction materials without the necessity of special solvents or additives. Accordingly, these two methods are advantageous in minimizing the kind of reaction materials to be used.
Moreover, it has been found that when an intermediate product of the metal addition/sulfurization reaction is reacted with carbon dioxide, the presence of water provides undesirable situations such as deterioration of the stability of a final product and prevention of smooth progress of the carbonation reaction (as disclosed in JP-B-3-10611). Contrary to these conventional findings, the present inventors have found that when excess dihydric alcohol is removed from the intermediate product and then the resulting intermediate product is reacted with carbon dioxide in the presence of an appropriate amount of water, the gram equivalent ratio of an alkaline earth metal per phenol component of an over-based phenate product (hereinafter referred to as "PEQ (product equivalent) value") is significantly improved (as disclosed in JP-A-5-238976 and JP-A-6-65192). (The term "JP-A" as used herein means an "unexamined Japanese patent application".) The improvement of PEQ value results in a reduction of the amount of expensive alkylphenols with a great deal of economical effect, since, in general, when additives such as a phenate are added to lubricating oil, the addition amount thereof is calculated based on the base number.
Furthermore, JP-A-6-184581 proposes decreasing the viscosity of over-based sulfurized alkaline earth metal phenates by lowering a sulfur content to be used.
However, in the methods of JP-B-60-28878 and JP-B-3-10611, it is extremely difficult to produce over-based sulfurized alkaline earth metal phenates having higher base numbers than conventional ones, and even though a product having a high base number is produced, the product has an extremely high viscosity, whereby it is disadvantageous in handleability. Moreover, JP-A-5-238976, JP-A-6-65192 and JP-A-6-184581 make it possible to improve the base number of over-based sulfurized alkaline earth metal phenates, but they are disadvantageous in that the color is dark and the oil solubility is poor.
Also, there is an example in which over-based sulfurized alkaline earth metal phenates are produced by adding fatty acids. For example, JP-A-58-225197 discloses that an alkylphenol, sulfur, a dihydric alcohol and an alkaline earth metal acid compound are reacted in an alcohol solvent and then in the subsequent carbon dioxide treatment, an organic carboxylic acid is added thereto in an amount of 0.1 to 10% based on the total amount thereof during or after the reaction to produce lubricating oil additives (over-based sulfurized alkaline earth metal phenates). The lubricating oil additives obtained by this method are improved in the viscosity, are prevented from foaming, and are improved in anti-sedimentation. In this method, however, the base number of the final product has as large as 250 mgKOH/g and it is impossible to further improve the base number, thus this is merely an example for lowering the viscosity of saturated fatty acids and for preventing the foaming.
Furthermore, JP-W-1-501399 and JP-W-1-501400 discloses producing a product having a base number of more than 300 mgKOH/g and a viscosity of 1,000 cSt or lower at 100.degree. C. by incorporating 2 to 40 wt % of carboxylic acids based on the total amount of phenates to conduct a metal addition reaction once or plural times. (The term "JP-W" as used herein means an "unexamined published International Japanese patent application"). In this method, however, the metal addition reaction must be repeated twice or more to produce a product which satisfies desired two performances at the same time; that is, the high base number and the low viscosity. JP-W-1-501399 and JP-W-1-501400 make it possible to improve the base number by the addition of fatty acids, but since they require plural metal addition reactions, which is disadvantageous in that the number of steps and the production costs are increased. In addition, JP-A-58-225197 improves the viscosity and the foaming resistance, but a product having a higher base number than those conventional ones cannot be obtained.